This invention relates to methods of manufacturing electronic devices comprising thin-film circuitry, in which a semiconductor film on a polymer substrate is subjected to a laser treatment (for example for crystal growth in the film). The device may be a large area image sensor, or a flat panel display (for example a liquid crystal display), or several other types of large-area electronic device (for example a thin-film data store or memory device, or a thermal imaging device). The invention also relates to laser apparatus suitable for use in such methods.
There is currently much interest in developing thin-film circuits with thin-film transistors (hereinafter termed "TFT"s) and/or other semiconductor circuit elements on inexpensive insulating substrates for large area electronics applications. These circuit elements fabricated with separate semiconductor islands of an amorphous or polycrystalline semiconductor film may form the switching elements in a cell matrix, for example in a flat panel display as described in U.S. Pat. No. 5,130,829 (our reference PHB 33646), the whole contents of which are hereby incorporated herein as reference material.
Recent developments involve the fabrication and integration of thin-film circuits (often using polycrystalline silicon) as, for example, integrated drive circuits for such a cell matrix. In order to increase the circuit speed, it is advantageous to use semiconductor material of good crystal quality and high mobility for thin-film islands of the TFTs of these circuits. From, for example U.S. Pat. Nos. 4,059,461 and 4,309,225 and the journal articles of S. D. Brotherton, D. J. McCulloch et al in IEEE Transactions on Electron Devices, Vol. 40 No. 2, February 1993, pages 407 to 413 and in Solid State Phenomena vols. 37-38 (1994) pages 299 to 304 (Scitec Publications, Switzerland), it is known to deposit a semiconductor film of amorphous material or low crystallinity material and to form the material of higher crystallinity by exposing, at least an area of this film to an energy beam from a laser. The whole contents of said United States patents and said journal articles are hereby incorporated herein as reference material. It is also known to use laser beam treatments for annealing implanted dopant and/or diffusing dopant in a semiconductor film in thin-film circuit manufacture.
As shown in said United States patents and said journal articles, the substrate with the deposited film is mounted within a sample chamber (also termed "cell") having a transparent window through which the laser beam enters. The cell provides a controlled atmospheric ambient (for example an inert gas ambient or vacuum) around the substrate/film sample. Generally the substrate/film sample is scanned below the laser beam, and this scanning may be achieved by mechanically moving the sample cell in the laser apparatus.
For many of the large-area electronic devices it is becoming desirable to use polymer material as the substrate for reasons of low cost, low weight and/or physical flexibility. The deposition of amorphous silicon material can be carried out at lower temperatures than the deposition of crystalline material and so is advantageous having regard to the maximum permissible temperatures for use with polymer substrates. These temperatures for polymers are generally lower than the maximum usable temperatures for glass substrates. Polyimide is one popular polymer material for the substrate of a large-area electronic device. The maximum usable temperature for polyimide is generally about 280.degree. C. The maximum usable temperature for a polymer material refers to the maximum sustained temperature at which the polymer material can be held for a prolonged period and retain its polymer characteristics, for example its flexibility and electrically insulating properties. Examples of other suitable polymer materials which may be used for the device substrate, together with their maximum usable temperatures are:
______________________________________ Polyethersulphone (PES) 200.degree. C. Polyacrylate (PAR) 180.degree. C. Poyetherimide (PEI) 170.degree. C. Polyethyelenenapthalate (PEN) 150.degree. C. Polyethyeleneterepthalate (PET) 130.degree. C. ______________________________________
As the semiconductor film can reach very high temperatures (for example, in excess of 1200.degree. C.) in the laser treatments, the film is generally deposited on one or more interface layers on the polymer substrate. These interface layers may be of (thermally and electrically) insulating material and/or material absorbing any laser light which penetrates through the thickness of the semiconductor film and/or can improve the adhesion of semiconductor film to polymer substrate. In spite of such measures, the present applicant finds that the laser treatment of semiconductor films on polymer substrates (as compared with glass substrates) tends to result in less uniform semiconductor material which is less satisfactory for device manufacture. For example, when the laser treatment serves for crystal growth, the present applicant finds that significant non-uniformities in the crystallised material may be produced along the scanned direction(s) of the film, in spite of consistent uniformity in the laser beam during scanning. These non-uniformities include differences in grain size and quality and a roughening of surface areas of the semiconductor film, the extent of the non-uniformities being sufficient to result in different device characteristics in the thin-film devices formed with different film areas. Furthermore, areas of the semiconductor film may become detached from the substrate and/or ablated, and areas of the polymer substrate may even exhibit heat damage, in spite of the inclusion of interface layers which should prevent such happenings.
It is an aim of the present invention to avoid or at least to reduce such non-uniformities and damage.
The present invention is based on a recognition by the present inventor that local distortion of the scanned polymer substrate at the area of the laser beam may occur (at least temporarily) due to substrate heating and can be a contributory factor in causing the damage and the poor uniformity in the semiconductor film laser-treated on polymer substrates. The present inventor has discovered that this local distortion may even focus the laser light reflected from the substrate/film sample which, if then reflected back again to the same local area of the film, can produce overheated spots. The present invention avoids this cause of poor uniformities by directing away the reflected laser light so that it is not reflected back again on to the film, or at least not reflected back again onto the same local area of the film.
Thus, according to one aspect of the present invention, in a method of manufacturing an electronic device a semiconductor film on a polymer substrate is subjected to a laser treatment from a laser beam, the beam being transmitted through a window over the film, the beam being reflected by the film, and the window and the substrate being inclined with respect to one another in the path of the beam to prevent the reflected beam from impinging again on the film.
According to another aspect of the present invention, there is provided laser apparatus suitable for use in such a method and comprising a cell for accommodating a polymer substrate with a semiconductor film for a laser treatment, and mounting means in the cell for mounting the substrate for exposing the film to a laser beam through a window of the cell, wherein the window of the cell and the mounting means for the substrate are inclined with respect to one another in the path of the beam from the window to prevent the reflected beam from impinging again on the film.
This prevention of back reflection onto the film from the window of a cell may be achieved by tilting the window and/or by tilting the substrate itself at least in that area. The perpendicular to the window and/or the substrate in the area of the beam path may be inclined at an angle in the range of 20.degree. to 60.degree., and preferably in the range of about 35.degree. to about 45.degree.. The maximum usable angle depends on the height available in the laser apparatus to accommodate the increased height of the cell and also depends on reducing reflection of the laser beam at the front face of an inclined window and/or on the depth of the focus of the laser beam at an inclined substrate. The minimum usable angle depends on whether one or both of the windows and substrate are inclined and depends on obtaining adequate deflection of the reflected beam away from the area of the film/substrate exposed to the laser beam. What constitutes adequate deflection can depend on the internal shape of the cell and the particular area(s) on which the reflected beam impinges. Small angles (for example in the range of 20.degree. to 30.degree.) can be used when both the window and the substrate are inclined in opposite directions to the path of the beam.
It may be noted that it is known from the JP-A 03-62924 English-language abstract in the Patent Abstracts of Japan Vol.15 No.219 (E-1074) (4747) to prevent the effect of returning light on laser output by passing the beam through one or more inclined glass plates (termed "substrates"). These plates are arranged between the laser light source and a total reflection mirror which directs the laser beam onto the sample undergoing the laser treatment. The plates have an anti-reflection coating on the side facing the laser light source being protected, so that the laser light is not reflected on that side. The opposite side (i.e. facing the total reflection mirror) has no anti-reflection coating, and the angle of inclination of the plate to the beam is such that the beam reflected at this opposite side is directed away from the path of the incident beam. Such an arrangement is quite unlike that adopted in accordance with the present invention in which measures are taken to prevent a local overheating effect of the laser light on the sample.
The present invention avoids such overheating while permitting laser treatments to be carried out on a sample which comprises a device substrate of heat-distortable polymer material, even in a particularly acute case in which the polymer material experiences (at least temporarily) a concave distortion in the substrate at an area where a semiconductor film on the substrate is heated by the incident laser beam. The beam is incident through a window over the sample, and the laser beam is focused as well as reflected by the concave distortion in the substrate. However, this focused and reflected laser beam is prevented from being reflected again onto the area of the film heated by the laser beam, because the facing window and/or the sample at the area of the concave distortion are tilted so as to be inclined with respect to one another.
Such an arrangement in accordance with the present invention does not require an anti-reflection coating to be provided on the obliquely facing surfaces of the window and the substrate/film, and so the problems of providing and maintaining anti-reflection coatings on these surfaces can be avoided. Thus, for example, an anti-reflection coating on the inner surface of a cell window can become contaminated and damaged by reactants and residues of a laser treatment carried out in the cell, and frequent cleaning and/or replacement of this coating can be both inconvenient and expensive. The provision of an anti-reflection coating on the sample may interfere with the desired treatment and/or characteristics of the film structure on the substrate. Thus, for example, the surface of a semiconductor film which is laser-crystallised with an anti-reflection coating thereon may exhibit severe roughening because of the presence of this coating.